This invention relates to noise suppression devices used with jet engines, and more particularly to a deployable, segmented exhaust nozzle for attenuating the noise produced by a jet engine.
With present day jet aircraft, structure typically known in the industry as xe2x80x9cchevronsxe2x80x9d have been used to help in suppressing noise generated by a jet engine. The chevrons have traditionally been fixed (i.e., immovable), triangular, tab-like elements disposed along a trailing edge of a secondary exhaust nozzle of the jet engine such that they project into the exhaust gas flow stream exiting from the secondary exhaust nozzle. The chevrons have proven to be effective in reducing the broadband noise generated by the mixing of primary-secondary and secondary/ambient exhaust streams for a wide range of operating conditions. Since the chevrons interact directly with the exhaust flow, however, they also generate drag and loss of thrust. Consequently, there is a tradeoff between the need to attenuate noise while still minimizing the loss of thrust due to the presence of the chevrons.
Noise reduction is typically needed for takeoff of an aircraft but not during cruise. Thus, any noise reduction system/device that reduces noise at takeoff (i.e., a high thrust condition) ideally should not significantly degrade the fuel burn during cruise. A compromise therefore exists between the design of static (i.e. immovable) chevrons for noise abatement and the need for low cost operation during cruise.
Thus, there exists a need for a noise reduction system which provides the needed noise attenuation at takeoff but does not produce drag and a loss of thrust during cruise conditions. More specifically, there is a need for a noise reduction system which permits a plurality of chevrons to be used in connection with an exhaust nozzle of a jet engine to attenuate noise during takeoff, but which also permits the chevrons to be moved out of the exhaust gas flow path of the engine during cruise conditions to prevent drag and a consequent loss of thrust during cruise conditions.
The above limitations are overcome by a noise reduction system in accordance with preferred embodiments of the present invention. In one preferred form the noise reduction system comprises a plurality of exhaust flow altering components spaced apart from one another and extending from a lip of an exhaust nozzle of a jet engine adjacent a flow path of an exhaust flow emitted from the exhaust nozzle. Each of the exhaust flow altering components are constructed to be controllably deformable from a first position adjacent the flow path to a second position extending into the flow path of the exhaust flow in response to a control signal applied to each of the flow altering components. In the first position, the flow altering components either have no affect on the thrust produced, or increase the momentum (thrust) of the exhaust flow exiting from the exhaust nozzle. In the second position, that is, the xe2x80x9cdeployedxe2x80x9d position, the flow altering components are deformed to extend into the flow path. In this position the flow altering components promote mixing of the exhaust flow with an adjacent air flow. This results in the attenuation of noise generated by the jet engine.
In one preferred form each of the flow altering components comprises a heat sensitive layer of prestressed, shape-memory material which responds to the exhaust flow (i.e., the control signal) by deforming such that it bends to project into the exhaust flow path when in the second position. In one preferred embodiment the shape-memory material comprises an alloy of nickel and titanium.
In another preferred embodiment a conductor is included in the flow altering component which allows an electrical current (i.e., the control signal) to be flowed through the flow altering component. The electrical current generates the heat needed to deform the flow altering component so that it can be moved into the second position.
In the above described embodiments, a second piece of material also is disposed adjacent the layer of prestressed, shape-memory material to act as a return xe2x80x9cspringxe2x80x9d. The second layer of material assists in returning the shape-memory material into the first position when the control signal is removed therefrom.
In the above-described embodiment which relies on the heat generated by the exhaust gas flow, the level of heat experienced during takeoff is sufficient to effect the deformation, and thus the deployment, of the flow altering components. As the aircraft reaches a cruise altitude, the significant cooling experienced by the flow altering components allows the flow altering components to be returned to their non-deformed (and thus non-deployed) orientations coinciding with the first position described above.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiments of the invention, are intended for purposes of illustration only and are not intended to limited the scope of the invention.